CHAPTER IV 



MODES OF APPLICATION 



Introductory Scientific Work on Effects of the 

 Synthetic Growth- Substances 



The rapidity with which the subject of synthetic growth- 

 regulating substances has developed can be judged from the 

 relative poverty of mentions of these substances in the first 

 edition of Boysen Jensen's book, which was published in 

 1936. This book is written mainly from the point of view of 

 the academic plant physiologist, rather than from that of the 

 horticulturist. To the chemist it will be of interest mainly on 

 account of the extent of the lists of references to work on the 

 physiological aspects of plant-growth stimulation; most of 

 these, however, are of the "pre-indole-acetic era", having 

 served to convey the ideas of botanists and plant physiologists 

 at a time when the chemistry of plant-growth stimulants was 

 non-existent. 



Physiological Methods of Testing 



The earliest methods of testing for the presence of growth- 

 promoting substances other than fertilizers were based upon 

 the amount of curvature occurring in a coleoptile (first leaf 

 sheath) of an Avena (oat) seedling after the growing tip had 

 been cut off and material containing the substance had been 

 applied to one side of the cut surface. This material was often 

 derived from another growing plant. The test depended upon 

 the stimulation of cell-elongation, and hence of growth on the 

 treated side, so that the cell-tissue became unequally devel- 

 oped, and bending was visible to the naked eye. The method 

 was suited to the detection of very small amounts of active 



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